1. Field of the Invention
The present invention relates to an eyepiece system and, in particular, to an eyepiece system having a high magnification which is suitable for use in an endoscope.
2. Description of the Related Art
An eyepiece system for use in an endoscope is an optical system which enlarges images sent from a leading end of an insertion portion of the endoscope through optical fiber bundles to an eyepiece portion of a hand control section of the endoscope so that the image can be observed. Since a surface to be observed thereof, that is, the end surface of the optical fiber bundles is generally very small in size, which an eyepiece system has a low ocular magnification, then the image observed by means of such eyepiece system is too small to observe, making it difficult for proper inspection and diagnosis to be executed by such endoscope.
This means that the larger image is necessary for observation and, therefore, it is essential that the focal length of an eyepiece system is made shorter.
Also, for easier observation of image from an eyepiece system, it is necessary that the distance (the eye point distance) from the eyepiece system to the pupil position of an observer is or make long.
Further, in order for the whole eyepiece system to be small in size, light in weight and inexpensive, it is desired that the eyepiece system is simplified in structure with a smaller number of components.
As an eyepiece system which is developed in consideration of the above-mentioned requirements, there are known eyepiece systems which are respectively disclosed in Japanese Patent Application Laid-open (Tokkai) No. 61-48809 and Japanese Patent Application Laid-open (Tokkai) No. 1-108516.
However, these eyepiece systems are those which are used in finder optical systems of electronic still cameras and also require a back focal distance, respectively. In general, when the combined focal length of a whole eyepiece system is designated as f, then the eyepiece magnification can be expressed by 250/f. According to the above-mentioned eyepiece systems, due to the fact that the eyepiece magnification thereof is in the order of 11 to 20 times, if such an eyepiece system is used for an endoscope, only a very small image can be observed through the eyepiece. Also, since such a system has only an short eye point distance ranging from 0.8 f to 0.9 f, it is not easy for an observer to observe the image through the eyepiece.